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1 Pharmacology and Physiology, University of Medicine and Denstistry of New Jersey - Graduate School of Biomedical Sciences, Newark, NJ, USA
* To whom correspondence should be addressed. E-mail: reeves{at}umdnj.edu.
Allosteric regulation by cytosolic Ca2+ of Na+/Ca2+ exchange activity in the Ca2+ efflux mode has received little attention because it has been technically difficult to distinguish between the roles of Ca2+ as allosteric activator and as transport substrate. Here we used transfected Chinese hamster ovary cells to compare the Ca2+ efflux activities in non-transfected cells and cells expressing either the wild-type exchanger or a mutant, 
(241-680), that operates constitutively, i.e. its activity does not require allosteric Ca2+ activation. Expression of the wild-type exchanger did not significantly lower the cytosolic Ca2+ concentration ([Ca2+]i) in comparison to non-transfected cells. During Ca2+ entry through store-operated Ca2+ channels, Ca2+ efflux by the wild-type exchanger became evident only after [Ca2+]i approached 100 - 200 nM; a subsequent decline in [Ca2+]i was observed, suggesting that the activation process was time-dependent . In contrast, Ca2+ efflux activity was evident under all experimental conditions in cells expressing the constitutive exchanger mutant. After a transient exposure to elevated [Ca2+]i, the wild-type exchanger behaved similarly to the constitutive mutant for tens of seconds after [Ca2+]i had returned to resting levels. We conclude that Ca2+ efflux activity by the wild-type exchanger is allosterically activated by Ca2+, perhaps in a time-dependent manner, and that the activated state is briefly retained following the return of [Ca2+]i to resting levels.
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